The invention relates generally to semiconductor device and integrated circuit fabrication and, in particular, to fabrication methods and device structures for heterojunction bipolar transistors.
A bipolar junction transistor is a three-terminal electronic device that includes an emitter, an intrinsic base, and a collector in its construction. The intrinsic base is arranged between the emitter and collector in the structure of the device. In an NPN bipolar junction transistor, the emitter and collector may be composed of n-type semiconductor material, and the intrinsic base may be composed of p-type semiconductor material. In a PNP bipolar junction transistor, the emitter and collector may be composed of p-type semiconductor material, and the intrinsic base may be composed of n-type semiconductor material. In operation, the base-emitter junction is forward biased, the base-collector junction is reverse biased, and the collector-emitter current may be controlled with the base-emitter voltage.
A heterojunction bipolar transistor is a variant of a bipolar junction transistor in which at least two of the collector, emitter, and intrinsic base are constituted by semiconductor materials with different energy bandgaps, which creates heterojunctions. For example, the collector and/or emitter of a heterojunction bipolar transistor may be constituted by silicon, and the base of a heterojunction bipolar transistor may be constituted by a silicon-germanium (SiGe) alloy, which is characterized by a narrower band gap than silicon.
Improved fabrication methods and device structures for heterojunction bipolar transistors are needed.